Cells in suspension vs. plated

I'm working with HEK293F cells and am using them for BRET experiments. I tend to get a much stronger signal when the cells are plated 24 hours in advance of BRET readings on a poly-d-lysine treated plate compared to when they're read in suspension. Does anyone know why?

This is just a (hopefully educated) guess but I think it will be because the optics are probably at the bottom of the plate, so there is no potential interference from the medium when taking the reading. Also, the attaached cells will be more spread out, rather than spherical, so there will be a larger area of cytoplasm visible for the interaction to be taking place.